Conventionally, projection type image display devices such as projectors have been used for obtaining large-screen images in an efficient manner. In projection type image display devices, liquid crystal panels or other spatial light modulation elements are used to form images in response to a picture signal and are illuminated with light from a lamp, so that their optical images are projected in an enlarged form upon a screen using a projection lens.
However, the use of a lamp as the light source is accompanied by a variety of problems, including: (1) short useful life and burdensome maintenance of the light source; (2) the complicated optical system required in order to separate white light into the three primary colors of light; and (3) a narrow color reproduction range.
In order to overcome these problems, projection type image display devices have been proposed, in which laser light sources are used instead of lamps. The useful life of laser light sources is longer than that of a lamp, and furthermore, due to its high directivity, the light usage efficiency of laser light is higher as well. In addition, its monochromaticity allows for a wide color reproduction range.
However, due to the high coherency, use of laser light is associated with the problem of speckle noise generation that causes image quality deterioration. Speckle noise is generated as a result of interference between light bundles in the image plane in accordance with complex phase relationships due to the scattering of phase-aligned waves emitted from laser light sources by rough object surfaces. Speckle noise is observed as an irregular granular intensity distribution.
If speckle noise appears in an image display device, viewers recognize it as degraded image quality. For this reason, when providing a laser light-based projection type image display device, it is extremely important to remove or reduce speckle noise. Various methods have been proposed for reducing such speckle noise (Patent Documents 1-5).
Patent Document 1 discloses reduction of speckle noise by effecting lasing action using an expanded spectral width. Further, Patent Document 2 discloses obtaining similar effects using a plurality of light sources of different wavelengths. However, in light sources with a narrow spectral width, such as in the case of green laser light obtained by means of wavelength conversion, practical application of the above presents difficulties.
Further, Patent Document 3 discloses separating laser light into a P-polarized component and an S-polarized component, combining them while introducing a light path difference, and impinging the laser light upon spatial light modulation elements. However, this method is difficult to apply when the light incident upon the spatial light modulation elements must be linearly polarized, such as when using liquid crystal light valves.
In another approach, various uncorrelated speckle patterns changing with time are generated and human-perceived speckle noise is reduced due to a superimposition effect. Patent Document 4 discloses generating various speckle patterns by subjecting the phase of light projected onto a screen to temporal modulation by vibrating optical fibers. Patent Document 5 discloses rotating fly-eye lenses about the axes of laser beams so as to change the angle of incidence of light upon spatial light modulation elements and generate a variety of speckle patterns.    Patent Document 1 JP 2002-323675A    Patent Document 2 JP 2004-503923A (Tokuhyo)    Patent Document 3 JP 2001-296503A    Patent Document 4 JP 2003-156698A    Patent Document 5 JP H11-064789A